Migration from OC10 to NC12

[mikevh1997]

To fix this issue, I did a manual migration. This worked perfectly:

@tflidd

make a full backup (all files from data, the database, config/config.php)
delete all code files (only keep data/ and config/-folder)
Download and extract code from Nextcloud server changelog

Fix permissions that webserver user can access files (Debian/Ubuntu: chown -R www-data:www-data /var/www/nextcloud)
Run upgrade from command line (Debian/Ubuntu: sudo -u www-data php /var/www/nextcloud/occ upgrade)
In some cases you need to turn off maintenance mode: sudo -u www-data php /var/www/nextcloud/occ maintenance:mode --off